If your midlife marriage seems like a battleground, Maggie Scarf has some advice: just wait. If you gut it out till retirement approaches, the likelihood is that things will improve. The children will leave, depression will lift, job woes vanish. And, while there is no guarantee, seemingly intractable marital conflicts may well fade away or resolve.

This is the optimistic news Scarf brings us in September Songs: The Good News About Marriage in the Later Years, her follow-up to the bestselling Intimate Partners: Patterns in Love and Marriage (update published in September 2008). Scarf bases her observations on interviews with about 75 pseudonymous subjects, ages 50 to 75, as well as research indicating that older people tend to live more in the present and have more stable emotions.

Take the case of the McBrides. Wife Lynn, having felt chronically unappreciated, tells Scarf that she used to assume that when her children grew up, she would leave her troubled marriage. But when husband Carl overcame his depression and finally dealt with a legacy of childhood sexual abuse, his involvement with Lynn deepened. As a result, her longtime anger ebbed. “It was time that had allowed for this profound transformation in the couple’s relationship,” Scarf writes.

Another couple, Jackie and Steve Winston, are still struggling with the aftermath of his attack of a rare disorder called transient global amnesia. And their transition to an active-adult community where nearly everyone else is retired hasn’t been easy. But their marriage has improved, in part because Steve has adjusted to Jackie’s perfectionism. “I don’t take something Jackie says as viscerally as I used to—whereas before I heard it as an assault on my character or some terrible defect of my being,” he tells Scarf.

While Scarf’s firsthand evidence is anecdotal, she supports her conclusion—that many marriages sweeten over time—with research from Stanford University’s Life-span Development Laboratory and elsewhere. “Older adults tend to live in the moment, and this appears to increase their satisfaction and well-being,” she writes. “It is this sense of time as a precious, diminishing resource that…lies at the source of the contentment, pleasure and satisfaction I encountered in so many of the over-50 partners I interviewed,” she says.

September Songs can be a slog at times, marred by leaden prose, repetition, and digressions on such topics as the decline of traditional pension plans. Scarf’s assertion that we are now living a full three decades longer than a century ago is also somewhat misleading. In fact, as Scarf herself seems to hint at one point, the 30-year rise in average life expectancy in the West is, in large part, the result of a huge reduction in child mortality.

Statistical anomalies aside, Scarf reports credibly that emotional intimacy and mutual tolerance in long-term marriages tend to rise. Something else, though, may not: most of Scarf’s couples report a marked lessening of sexual activity. In the interviews, they express little dismay on the subject and generally disdain the use of Viagra and its pharmaceutical cousins. (One wonders, given how profitable these drugs have been, if these interviewees are really typical.)

An otherwise harmonious couple, Jean and Ned Donaldson (ages 58 and 66, respectively), with fading desires for sexual intercourse, consider cuddling a great substitute. If sex is “hopping into bed and curling tightly next to each other, so close that you are almost melted into one body,” says Ned, “then we have lots of sex.” Even without youthful passion, Scarf concludes, “The Donaldsons were at peace, and in a place that was deeply satisfying to both of them.”

Julia M. Klein is a Philadelphia-based cultural reporter and critic and a contributing editor at Columbia Journalism Review. Read her review of Assisted Loving: True Tales of Double Dating with My Dad

Read more Web-exclusive book reviews.